Permeable membrane systems have been increasingly employed in various fluid separation process. In such separation processes, a fluid mixture is brought into contact with the surface of the membrane in order to permeate the more readily permeable component of the fluid mixture through the membrane. As the more readily permeable component is withdrawn as a permeate stream, the less readily permeable component of the fluid mixture is recovered or removed as a non-permeate stream.
Significant factors in the design and overall efficiency of membrane systems are the total membrane surface area required for a given fluid separation and the partial pressure difference across the membrane that is required to obtain a desired product quantity and quality, which reflect the membrane's permeability and selectivity (or separation factor) characteristics respectively. The design of practical membrane systems requires optimization of the trade-offs between membrane surface area and said partial pressure differences. Thus, the grater the partial pressure difference, or driving force, across the membrane, the less is the membrane surface area required for a given fluid separation. This necessitates the use of more expensive pumping equipment and higher pump operating costs, but enables membrane equipment costs to be kept relatively low. If, on the other hand, a lower driving force is employed, more membrane surface area is required, and the relative costs of the various aspects of the overall system and operation would change accordingly.
Membrane systems are usually designed and optimized for full capacity, steady constant flow conditions, i.e., design conditions, that are not always fully utilized in practice. Under operating conditions other than the design conditions, different combinations of optimum operating conditions will prevail with respect to membrane area versus partial pressure differences because fluid separation applications for which membrane systems are desirable typically do not run under steady flow conditions. The demand from the membrane system will often vary in terms of product quantity and/or quality. For example, product demand for nitrogen gas from an air separation membrane system can vary significantly in a twenty-four hour period in terms of nitrogen flow rate and/or purity required. This varying product demand dictates that the membrane systems be designed to operate efficiently during off demand or turn-down conditions.
Several techniques have been used or proposed in attempt to operate membrane systems efficiently during the off demand or turn-down conditions. U.S. Pat. No. 4,806,132 discusses a number of techniques, which have been previously employed to operate membrane systems during off demand or turn-down conditions. These previous techniques involve reducing the flow of a non-permeate product stream by increasing the permeation of a greater amount of a fluid feed mixture, by shutting down a portion of the available membrane surface area or by using a surge tank to unload the membrane system. Due to their inefficient use of the available membrane surface area and power, however, this patent decides to reduce the flow of a feed stream during the off demand or turn-down conditions to operate membrane systems efficiently. The flow of a feed stream into the membrane system is controlled by adjusting the operation of a feed compressor located at the inlet line of the membrane system with controlling means.
U.S. Pat. No. 4,857,082 discusses in columns 1 and 2, inter alia, U.S. Pat. No. 4,397,662 which discloses a technique for operating a membrane system under turn-down conditions. The technique is indicated to involve removing membrane area by automatically valving off a portion of the membrane modules at predetermined production levels. To achieve this result, the membrane system is indicated to utilize a complex design involving additional valves, piping, instrumentation, etc. This design is indicated to be capital intensive due to the use of several smaller modular membrane units in parallel. Thus, the intent of U.S. Pat. No. 4,857,082 is to use a series of valves and control units located around membrane units to effectively control the differential pressure across the membrane to follow the changes in product demand or feed flow. This scheme is particularly adaptable to processes where the permeate gas is the desired product.
None of the techniques discussed above, however, is directed to control systems and processes useful for multi-stage membrane systems having at least three membrane stages and at least one recycle compressor, which are capable of recovering a high purity non-permeate product stream. Such multi-stage membrane systems are becoming increasingly important since they could produce, for example, high purity nitrogen without employing a deoxo unit. Therefore, there is a genuine need for control systems and processes which are useful for operating multi-stage membrane systems having at least three membrane stages and at least one recycle compressor during off demand or turn-down conditions.